International Journal of Earth Sciences
Relationship between alternating open/closed conduit conditions and deformation
patterns: an example from the Somma-Vesuvio volcano (southern Italy)
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Full Title: Relationship between alternating open/closed conduit conditions and deformation patterns: an example from the Somma-Vesuvio volcano (southern Italy)
Article Type: Original Paper
Keywords: volcano-tectonics; structural analysis; DTM lineament analysis; volcanic dykes Corresponding Author: Francesco D'Assisi Tramparulo, Ph.D.
Istituto Nazionale di Geofisica e Vulcanologia ITALY
Corresponding Author Secondary Information:
Corresponding Author's Institution: Istituto Nazionale di Geofisica e Vulcanologia Corresponding Author's Secondary
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First Author: Francesco D'Assisi Tramparulo, Ph.D. First Author Secondary Information:
Order of Authors: Francesco D'Assisi Tramparulo, Ph.D. Stefano Vitale, Ph.D.
Roberto Isaia, Ph.D. Alessandro Tadini, Ph.D. Marina Bisson, Ph.D. Ernesto Paolo Prinzi, Dr Order of Authors Secondary Information:
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Abstract: With the aim to investigate the relationships between the current deformation setting of the Somma-Vesuvio volcano and its volcano-tectonic evolution, we show the results of a meso-scale systematic structural analysis on fractures, faults and dykes and a macro-scale morphological lineament analysis carried out from high-resolution Digital Terrain Models (DTMs). Fractures include: (i) cooling joints, subsequently reactivated as normal and oblique faults during the caldera collapse episodes; (ii) radial and tangential metric fractures likely associated to volcanic inflaction/deflaction stages and (iii) macro-fractures related to slope instabilities. Dykes occur with different geometries including en-echelon structures bounding grabens. The orientation analysis on all structures marks preferred directions NW-SE, NE-SW, N-S and E-W amongst the dominant ones. Furthermore, azimuth and spatial position of dykes and morphological lineaments were analyzed comparing them with the old Somma volcano and Gran Cono centers, respectively. Results highlight the overprinting of radial and clustered strain patterns recorded in different volcano-tectonic evolution stages: the radial pattern forms during the volcanic inflation with a closed magmatic conduit; on the contrary, the regional strain pattern, characterized by preferred orientation, prevails on the local deformation field in the case of the open conduit activity.
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Relationship between alternating open/closed conduit conditions and
1
deformation patterns: an example from the Somma-Vesuvio volcano
2
(souther Italy)
3
Tramparulo F.D.A.1, Vitale S.1-2, Isaia R.1, Tadini A.3-4, Bisson M.4, Prinzi E.P.2
4 5
1Istituto Nazionale di Geofisica e Vulcanologia, sezione di Napoli Osservatorio Vesuviano, Via 6
Diocleziano 328, 80124 Napoli
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2Dipartimento di Scienze della Terra, dell’Ambiente e delle Risorse (DiSTAR), Università di 8
Napoli Federico II, Largo San Marcellino 10, 80138, Napoli
9
3Dipartimento di Scienze della Terra, Università di Firenze, Firenze, Italy 10
4Istituto Nazionale di Geofisica e Vulcanologia, Sezione Pisa, Pisa, Italy 11
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Corresponding author: francescodassisi.tramparulo@unina.it, tel +39.081.2538124
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Keywords: volcano-tectonics, structural analysis, DTM lineament analysis, volcanic dykes.
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Abstract
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With the aim to investigate the relationships between the current deformation setting of the
Somma-16
Vesuvio volcano and its volcano-tectonic evolution, we show the results of a meso-scale systematic
17
structural analysis on fractures, faults and dykes and a macro-scale morphological lineament analysis
18
carried out from high-resolution Digital Terrain Models (DTMs). Fractures include: (i) cooling joints,
19
subsequently reactivated as normal and oblique faults during the caldera collapse episodes; (ii) radial
20
and tangential metric fractures likely associated to volcanic inflaction/deflaction stages and (iii)
21
Manuscript Click here to download Manuscript Manuscript.docx
2
macro-fractures related to slope instabilities. Dykes occur with different geometries including
en-22
echelon structures bounding grabens. The orientation analysis on all structures marks preferred 23
directions NW-SE, NE-SW, N-S and E-W amongst the dominant ones. Furthermore, azimuth and
24
spatial position of dykes and morphological lineaments were analyzed comparing them with theold
25
Somma volcano and Gran Cono centers, respectively. Results highlight the overprinting of radial and
26
clustered strain patterns recorded in different volcano-tectonic evolution stages: the radial pattern
27
forms during the volcanic inflation with a closed magmatic conduit; on the contrary, the regional
28
strain pattern, characterized by preferred orientation, prevails on the local deformation field in the
29
case of the open conduit activity.
30 31
3
1. Introduction
32
Defining deformation structure patterns in the volcanic edifices is a useful tool for disclosing the
33
stress field acting during the volcano activity and the role of the local stress field with respect to the
34
regional deformation pattern (e.g. Gudmundsson, 1995; Marinoni and Gudmundsson, 2000; Pollard
35
et al., 1983). Volcanoes characterized by a well-defined central conduit, such as the Somma-Vesuvio
36
(SV), are mainly characterized by radial deformation patterns of faults, fractures, dykes and eruptive
37
fissures (e.g. Acocella and Neri, 2009; Vezzoli et al., 2014) usually resulting from uniform stress
38
fields, whose symmetry is a consequence of the stress focusing above the magma chamber center.
39
According with several authors (e.g. McGuire and Pullen, 1989; Pinel and Jaupart, 2003) the main
40
factor driving this phenomenon is the edifice load. However, stress focusing is also controlled by the
41
presence of a well-localized central conduit that at SV is well-defined by geophysical surveys (Di
42
Stefano and Chiarabba, 2002; Zollo et al., 2002; De Natale et al., 2004). Nevertheless, discrepancies
43
from this simple model can exist. A strong tectonic stress field can considerably modify the radial
44
pattern (e.g. Muller and Pollard, 1977; Nakamura, 1977; Odé, 1957), for example in the case of
45
synchronous activity of a volcanic radial pattern and a well-marked regional deformation field, the
46
resulting strain field is a complex array of structures that can be disclosed by means of specific
47
orientation analyses that separate radial and unidirectional components (e.g. Quintà et al. 2012). In
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others cases the occurrence of mega-landslides (Becerril et al., 2015) or preexisting anisotropies in
49
the host rock (Gudmundsson, 2006) can locally mask the radial pattern. In the case of SV, a local
50
variation of the radial pattern could result by the active gravitational spreading triggered by the edifice
51
load and the occurrence of a weak sedimentary layer underlying the volcano (e.g. Borgia et al., 2005;
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Marturano et al., 2009; D’Auria et al., 2014) or by flank failures (e.g. Ventura et al., 1999).
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This paper aims to provide a complete framework of the deformation state of the SV volcano, by
54
means of a multi-scale approach that combines the orientation analysis of fractures, faults, dykes
55
(collected during several field surveys), and the analysis of morphological lineaments derived after
4
the inspection of high-resolution Digital Terrain Models (DTMs). This study allowed us to shed light
57
on the connection between the current deformation pattern and the complex tectonic and eruptive
58
history of the volcano, characterized by Plinian to effusive eruptions and alternating open and closed
59 conduit stages. 60 61
2. Geological setting
62The SV volcano is located in the southern sector of the Campanian Plain (Fig. 1), a structural
63
depression filled by Pleistocene to Holocene continental, volcanic and marine deposits, bounded to
64
SE, E and NE by several Meso-Cenozoic carbonate ridges (e.g. Santangelo et al., 2010 and references
65
therein). The SV is a stratovolcano ∼1300m high formed by lavas, scoriae and pyroclastic
66
depositsincluding ultrapotassic to potassic composition such as leucititic tephrites, phonolites and
67
trachytes (Santacroce, 1987; Barberi et al. 1981; Bernasconi et al. 1981; Santacroce et al. 2008).
68
Geophysical and borehole data indicate that volcanic activity of SV is subsequent to the emplacement
69
of the Campanian Ignimbrite eruption products (occurred at 39.8 ka; Giaccio et al., 2017), which
70
represents the major Plinian eruption of the nearby Campi Flegrei volcano recorded in the Campanian
71
Plain. The present shape of SV is characterized by a younger cone (Gran Cono) localized within an
72
about E-W oriented elliptical caldera, constrained to the N by an arc-shaped scarp cutting the old
73
Somma Volcano. . The SV multistage summit caldera (located between ca. 560 and 870 m a.s.l.), is
74
the result of caldera collapses (with different geometries) which accompanied the four major Plinian
75
eruptions (Cioni et al., 1999). The oldest Plinian eruption occurred at the SV volcano (i.e. the 18 ka
76
BP Pomici di Base eruption) produced a caldera collapse which truncated the early Somma Volcano,
77
whose crater center was located between the present Gran Cono and the Mt. Somma scarp (Fig.1).
78
Subsequently the last Plinian eruption at SV, occurred in 79 AD, the Gran Cono started to grow
79
discontinuously, with its present shape and structure resulting mostly from the 1631-1944 AD
80
eruptive activity. The oldest outcropping products of the SV volcano are exposed at the base of the
5
Mt. Somma scarp and date back to about 30 ka BP; while those belonging to the last cycle of activity
82
(1631-1944 AD) are located on top of the scarp itself (Santacroce and Sbrana 2003). Several dykes
83
crosscut the volcanic succession exposed along the caldera wall (Porreca et al., 2006); most of them
84
are sub-vertical, while others show dip directions both outward and inward with respect to the caldera.
85
It is widely accepted that the SV volcano is characterized by a well-localized magmatic conduit in
86
depth which has determine the whole volcanic activity including both small-scale effusive events and
87
large-scale explosive eruptions (Santacroce, 1987). Millennia long periods of rest, with a closed
88
conduit condition, preceded the largest eruptions (e.g. Rosi et al. 1993); on the contrary an open
89
conduit setting of the volcano is associated to a continuous moderate activity with small-scale
90
eruptions (e.g. Cioni et al. 2008).
91
The volcanic evolution after the 79 AD Plinian eruption alternated periods of open conduit condition
92
with semi-persistent mainly effusive activity and periods of closed conduit condition (the latter ones
93
preceding the sub-Plinian eruptions of 472 AD and 1631 AD; Rosi et al. 1993). During periods of
94
open conduit condition (e.g. S. Maria cycle and the period 1631-1944; see Cioni et al. 2008), the
95
volcanic activity was often associated with the formation of eruptive fissures (Bianco et al. 1998,
96
Ventura and Vilardo, 1999), such as those localized southward with a N-S direction (1760 AD) and
97
westward with a WSW-ENE direction (1697-1861 AD) (Fig.1). Major fissures are also localized in
98
the NW and NE flank with NW-SE and NE-SW directions, respectively, but with an older age of ca.
99
22-19 ka BP (Santacroce, 1987; Bianco et al. 1998; Santacroce and Sbrana 2003).Other minor
100
fissures, hosted in the Gran Cono flanks, ranging in age between 1701 and 1899 AD (Santacroce,
101
1987; Santacroce and Sbrana, 2003; Acocella et al., 2006; Cioni et al., 2008) and showing N-S, E-W
102
and WNW-ESE trends (Tadini et al., 2017).
103 104
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3. Structural analysis of the Somma-Vesuvio volcano
105
3.1 Fractures 106
The SV rocks host fractures, faults and volcanic dykes. Generally fractures in lava layers appear as
107
high dip-angle cooling joints (Fig. 2a, c), mostly orthogonal to the lava boundary surfaces and
108
forming two orthogonal sets (Fig. 2a). Cooling joints within volcanic dykes form roughly square and
109
hexagonal systems (columnar joints, Fig. 2c), with dip-angles ranging from horizontal to
sub-110
vertical, usually orthogonal to the host rock walls (Fig. 2c). Other fractures, with lengths of several
111
tens of meters, crosscut the Mt. Somma scarp, appearing as radial (Fig. 2d) and tangential (Fig. 2e)
112
respect to the caldera. Radial fractures also occur along the inner crater of the Gran Cono (Fig. 2f).
113
Two en-echelon macro-fractures crosscutting the 1929 AD pahoehoe lavas (Fig. 3c) along the eastern
114
rim of the caldera (Fig. 3a-c) are characterized by a N-S direction and a total length of ca. 72 m. The
115
easternmost fracture shows at least 1 m of horizontal displacement, with few centimeters of vertical
116
slip and an angular rotation of the hanging wall (Fig. 3c). The westernmost macro-fracture shows a
117
larger length but a minor opening (lesser than 1 m).
118
Furthermore, also ash and pyroclastic deposits, exposed in the quarries located along the SV flanks,
119
host fractures, usually forming high dip-angle orthogonal joint sets. These fractures are mainly
120
developed in fine grained deposits (e.g. compact ash layers) and less in coarse grained sediments (e.g.
121
pumice layers).
122
The orientation analysis of the cooling joints in lavas, indicates an almost uniform radial pattern for
123
the fracture directions, with a slight prevalence of the E-W and NE-SW trends (Fig. 4a). On the other
124
hand the orientation data of fractures, within pyroclastic rocks, reveal the occurrence of three main
125
trends: NE-SW, NW-SE and E-W (Fig. 4b). In order to investigate possible preferred orientations of
126
cooling joints, the area was subdivided in 200x200 m-sized cells. For each cell the main fracture
127
direction and the S3 eigenvector were calculated and analyzed by means of rose diagram and 128
7
stereographic projection, respectively, as illustrated in Figure 4. The S3 eigenvector indicates the 129
extension axis calculated through the Bingham method (Bingham, 1974). Fracture dominant
130
directions were calculated through bipolar rose diagrams characterized by an azimuth interval of 10°.
131
Figure 5a and 5b shows the fracture and S3 directions, respectively, represented in the map as oriented 132
segments. The rose diagram (Fig. 4c) of the main directions of all of fractures (cooling joints collected
133
in lavas and fractures in pyroclasts), marks the prevalence of E-W, ENE-WSW and NNE-SSW
134
directions. The stereographic projection of all S3 axes indicates a main cluster with a NNW-SSE 135
direction and a secondary cluster with ENE-WSW direction (Fig. 4d). In order to unravel a possible
136
difference between the strain pattern close and far from the Gran Cono center, the preferred
137
orientations of all the fractures were further analyzed subdividing the area in three concentric circular
138
sectors (Fig. 5a): (i) the inner circle includes all fractures hosted in the Gran Cono; (ii) the second
139
circular sector comprises the Mt. Somma scarp and finally (iii) the external ring includes the remnant
140
fractures. For each area, rose diagrams of the fracture main directions were calculated (Fig. 5e-g).
141
The Gran Cono area (Fig. 5a) show a main E-W direction and secondarily N-S, NE-SW and NW-SE
142
directions. For the second circular area (Fig. 5f) the orientation analysis marks the dominant
NNE-143
SSW and E-W directions whereas for the external sector (Fig. 5g) the main directions are ENE-WSW
144
and WNW-ESE. Further analyses of these structural features were carried out subdividing data in two
145
groups (A and B) according to the age of hosting deposits (< 79 AD and > 79 AD, respectively).
146
Resulting rose diagrams indicate that for group A (Fig. 5h), the main trends are NE-SW, E-W and
147
NW-SE, while for group B (Fig. 5i), in addition to the same trends of group A, also the N-S direction
148 occurs. 149 150 3.2 Dykes 151
Volcanic dykes, well-exposed along the Mt. Somma scarp and Gran Cono walls, are characterized by
152
variable length, thickness, orientation and geometry. A total of 100 dykes were measured and mapped
8
(Fig. 6a). The analyzed dykes occur as single (Fig. 3g), parallel (Fig. 3h) or en-echelon sets (Fig.
3d-154
f). Dykes are from sub-vertical to moderately and gently dipping both inside and outside the caldera,
155
somewhere as sills (Fig. 3g). Two en-echelon conjugate dyke sets bound an NE-SW striking graben
156
(Fig. 3d). In the western sector of the Gran Cono (Fig. 3h) a main N-S sub-vertical dyke and a parallel
157
minor structure crosscut deposits belonging to the period 1855-1872 AD (Santacroce, 1987). Dyke
158
orientation analysis indicates a dominant NE-SW direction (Fig. 6d); however, by normalizing the
159
data according to the length of the dykes, the NE-SW and NW-SE directions appear to be the main
160
ones (Fig. 6e). In order to unravel the relationship between the directions of the dykes located in the
161
Mt. Somma scarp (Fig. 6), we applied the methods illustrated by Porreca et al. (2006) and Quintà et
162
al. (2015). Assuming that the formation of these dykes is synchronous with the eruptive fissures
163
located on the NW and NE flanks, whose ages are older than the first caldera collapse (18 ka; Cioni
164
et al., 1999), we used the center of the old Somma volcano as bench mark for the radial analysis.
165
and angles were calculated, where: (i) is defined as the angle between dyke location and the
166
North (from 0° to 360°); (ii) is the strike of the dyke (from 0° to 180°) and (iii) is the absolute
167
value of the difference between and . The angle, ranging between 0 and 90°, allows to classify
168
the dykes in i) radial (≤ iioblique (30°≤≤60°) and iii) tangential (≥60°). It results that 59%
169
of dykes are radial, 24% oblique and only 17% tangential (Fig. 6c). The - contour diagram shows
170
both the main NE-SW trend and the radial pattern (Fig. 6b).
171 172
3.3 Faults 173
Most of the faults were observed along the Mt. Somma scarp. The cooling joints in the lava strata,
174
oriented parallel to the caldera scarp (tangential joints) generally show slicken-side steps and
175
striations (Fig. 2a): The latter are characterized by high values of pitch on the tangential planes
176
indicating a dominant normal kinematics whereas, locally on the radial planes, oblique kinematics
9
with pitch angles >45° occurs (Fig. 2a). The larger structures are tens of meters long high-angle
178
normal faults parallel to the caldera scarp showing displacements of few meters (Fig. 7b, c). Rare
179
low-angle faults also occur, whose normal kinematic component is inferred by dislocated planar
180
features such as preexisting dykes (Fig. 2d). These low-angle normal faults are generally crosscut by
181
sub-vertical radial fractures and faults (Fig. 2d). Moreover, low-angle normal faults show few
182
centimeter-sized thick damage zones, marked by a high fracture density (Fig 7a). With respect to the
183
pyroclastic sediments located along the SV flanks, they host few normal faults showing a
syn-184
sedimentary character, such as growing strata and plastic deformation in the hanging wall (Fig. 7e).
185
Slicken-side striations are very rare, locally they occur in scoria deposits always showing dip-slip
186
normal kinematics (Fig. 7d).
187
The stereographic projections of fault poles and the rose diagram of fault directions indicate a
188
dominant E-W direction (Fig. 8a). The rose diagram of the fault main directions, including both faults
189
in lavas and pyroclastic rocks, each one calculated for every cell, shows main E-W and N-S directions
190
and secondary NE-SW and NW-SE directions (Fig. 8b). By means of the P-B-T method (Angelier
191
and Mechler, 1977; Reiter and Acs, 1996–2003) we calculated the T-axes (extension) characterized
192
by NNE-SSW and NNW-SSE directions. Finally the maps of figures 5c and 5d show the fault main
193
directions and the T-axis directions, respectively.
194 195
4. Lineament analysis
196
With the aim to highlight possible connections between the volcano-tectonic activity and the
197
morphologic surface of the SV, lineament extraction has been obtained from the SV Digital Terrain
198
Model (DTM). Lineament analysis on DTMs was previously provided by different authors, with the
199
aim to study volcano-tectonic (Ventura et al., 1999) or geomorphological (Ventura et al., 2005;
200
Alessio et al., 2013) features. In this study we used a DTM with a higher ground resolution (1x1 m)
10
with respect the previous analyses. Elevation data of the SV topography have derived from two
202
different sources: i) the “Ufficio Sistema Informativo Territoriale (SIT)” of the “Provincia di Napoli”
203
(Project “Centro Satellitare Cave della Provincia di Napoli – Ce.CO.SCA”); ii) the “Ministero
204
dell'Ambiente e della Tutela del Territorio e del Mare (MATTM)” with license Creative Commons
205
3.0 Italy (CC BY-SA-3.0IT). These models were provided in 5451 ASCII files (3615 from 2009 flight
206
and 1836 from 2012 flight), matrices of 500 rows x 500 columns having a cell size of 1 meter and
207
geocoded into WGS84 UTM ZONE 33 reference coordinate system. The elevation data of the
208
MATTM instead were acquired during ALS flights occurred in 2008 year. Also this data were
209
provided in matrices with cell sizes of 1 m.
210
With the term “lineament” we mean here any natural linear pattern visible by the shaded reliefs of
211
the study area. Lineaments that clearly represent anthropic features (road cuts, terraces, buildings,
212
etc.; Fig. 9a) or irregularities in the DTM (due to acquisition errors), have been not included in this
213
investigation. For a correct collection of natural lineament data we have selected an area of ca. 71
214
km2 characterized by more smooth topography and weak urbanization. The investigation scale was 215
defined to 1:10,000. To extract natural lineaments two steps have been performed. For the first step,
216
all the clearly evident natural lineaments have been traced starting from 4 shaded reliefs obtained
217
illuminating the DTM with six different settings of azimuth from N (0°, 45°, 135°, 180°, 225° and
218
315°) and maintaining constant the illumination angle of 45°. As regards the second step, in order to
219
magnify possible directional trends, three different gradient filters (N, NE and NW) have been applied
220
to all the shaded reliefs (in order to enhance the visibility of the features with directions opposite to
221
that of filter itself) combining a directional 3x3 convolution kernel to each matrix of the shaded
222
reliefs. Following the two steps described above, a database of 4095 natural lineaments was
223
implemented (Fig. 9b). Each lineament has been analyzed from the point of views of its (i) length
224
(expressed in meters); (ii) azimuths (expressed in degrees with respect to N) and (iii) spatial locations.
225
Furthermore the (iv) orientation analysis with respect the volcano center was calculated as well. The
226
analysis method is the same of that previously used for the dykes. Assuming that the lineaments are
11
mostly related to the recent activity of the SV volcano, the Gran Cono center was selected for this
228
analysis. In order to analyze the relationship between lineament orientations and distance from the
229
Gran Cone center, six different circular areas have been identified with radii ranging between 0-1,
230
0.5-1.5, 1-2, 1.5-2.5, 2-3 and 3-4 km (Fig. 10). We identified more circular areas with respect the
231
fracture orientation analysis, because of the large amount of data uniformly distributed in the study
232
area.
233
The - contour plots (Fig. 10) show that in each pattern, a superposition between radial and regional
234
configurations occur, the latter marked by lineament clustering characterized by N20-30E and
N70-235
80E directions (), especially for radii lesser than 2.5 km (comprising the Mt. Somma scarp). The
236
two regional directions (about NNE-SSW and ENE-WSW) are well-marked in the rose-diagrams
237
associated to each circular area (Fig. 10). Finally, -frequency histograms (Fig. 10) confirm that the
238
most part of lineaments are radial, followed by oblique and only a secondarily tangential.
239 240
5. Discussion
241
The analyzed fractures can be grouped in three main types: i) decimeter-sized cooling joints hosted
242
in lavas, sills and dykes; ii) meter to tens of meters in length tangential/radial fractures, crosscutting
243
different lava layers, scoriae and pyroclastic products; iii) N-S macro-fractures located to the eastern
244
rim of the Mt. Somma scarp, crosscutting the 1929 AD pahoehoe lavas.
245
The orientation analysis on the whole dataset of the cooling joints, reveals a random direction pattern,
246
on the contrary when analyzed for each 200x200 m cell, preferred orientations result. These fracture
247
preferred azimuths show dominant N-S, E-W, NE-SW and NW-SE directions. Furthermore, the
248
orientation analysis, performed for circular areas centered in the Vesuvio crater, and including all
249
fractures (both in lavas and in pyroclasts) indicates that the N-S and E-W fracture sets are mostly
250
developed close to the Gran Cono and Mt. Somma scarp (0-2 km of radius), whereas the NE-SW and
12
NW-SE directions are dominant for the external sector (>2 km). Grouping the fracture data by age of
252
the hosting deposits (group A with ages < 79 AD and group B with ages > 79 AD), results that group
253
A fractures show trends NE-SW, E-W and NW-SE; whereas in younger deposits of group B a further
254
N-S trend is present.
255
The occurrence of preferred orientations for the cooling joints can be related to (i) a synchronous
256
volcano-tectonic stress field (Hancock and Engelder, 1989) or ii) an articulated topography modelled
257
by preferentially oriented volcano-tectonic structures (Fleischmann, 1991). In any case these features
258
suggest that cooling joint pattern is controlled by a preexisting or simultaneous volcano-tectonic
259
deformation field.
260
As concerning the radial and tangential fractures, they generally form during the volcanic inflation
261
and they can be reactivated as faults during a deflation stage accompanied by a caldera collapse (e.g.
262
Martì et al., 1994). Finally, the open macro-fractures can be related to the ground instability along the
263
volcano slope. This is validated by the occurrence of tilted hanging walls suggesting that the shallow
264
sub-vertical fracture planes downward pass to listric surfaces, typical of the rotational landslides.
265
The main result of the dyke orientation analysis, carried out in this study, indicates a dominant radial
266
pattern for these structures in agreement with Porreca et al. (2006). However, when considering the
267
preferred orientations, dykes appear to form two main sets with N115E and N50E directions. These
268
orientations are also similar to those of the two eruptive fissures located, respectively, in the NW
269
sector (strike N128E) and NE sector (strike N48E), whose ages are both comprised between 22 and
270
19 ka (Santacroce, 1987 and Bianco et al. 1998). Probably this temporal interval corresponds to that
271
of most of the dyke formation, presently exposed along the Mt. Somma scarp. On the other hand, the
272
dykes within the Gran Cono, characterized by N-S direction, are hosted in the 1855-1872 AD rocks
273
(Santacroce, 1987). The simultaneous radial and clustered array (anisotropic radial pattern; Acocella
274
and Neri, 2009) of the dyke direction is characteristic of several other volcanoes around the world
13
such as Etna (Acocella and Neri, 2003) and Stromboli (Tibaldi 2003) in Italy, Fuji in Japan (Takada
276
et al., 2007) and Erta Ale in Ethiopia (Acocella, 2006).
277
The geometry of the dykes is generally planar, though several dykes located in the Mt. Somma depict
278
a conjugate en-echelon array. This geometry suggests that the magma flow within the en-echelon
279
apophyses was sub-horizontal, such as stated by Porreca et al. (2006) by means of the Anisotropy
280
Magnetic Susceptibility study. Probably magma flowed within weakness bands bounding a graben
281
structure suggesting a shear component with a normal kinematics for the emplacement of the
en-282
echelon dykes. The NE-SW direction of both dominant dyke swarm and en-echelon dyke graben, 283
suggest a main extension characterized by a NW-SE direction, at least during their emplacement
(22-284
19 ka BP).
285
According to Porreca et al. (2006), this configuration occurred with an open volcanic conduit stage
286
(Fig. 11a). On the contrary the closed conduit condition increases the magma pressure triggering the
287
volcano inflation and the prevalence of the radial field with the formation of both radial and tangential
288
fractures (Fig. 11b).
289
Such as observed in several volcanic settings (e.g. Acocella, 2007 and references therein), the
290
development of ring faults is related to caldera collapse processes. At SV, these structures crosscut
291
the whole volcanic sequence; however, they worked at the meso-scale, reactivating preexisting planes
292
including cooling joints. All the slicken-side striations observed on the tangential planes indicates a
293
normal dip-slip kinematics, whereas the radial faults show an oblique kinematics. We conclude that
294
during caldera collapse stages, tangential and radial fractures previously formed as result of the
295
volcanic inflation or as cooling joints, reactivate as (i) normal ring faults inward dipping to the caldera
296
and (ii) transfer faults, respectively (Fig. 11c). In this case transfer faults have allowed that different
297
blocks independently move toward the center of the collapsing caldera.
14
Another type of structure is represented by low-angle normal faults. Despite their length, they show
299
small displacements, crosscutting dykes and in turn cut by radial fractures and ring faults. Probably
300
low-angle normal faults are related to the inflation stage that triggered gravitational instabilities along
301
the volcano flanks (Fig. 11b).
302
Faults were observed also along the volcano slopes crosscutting pyroclastic deposits. They are
303
frequently associated to plastic deformation features suggesting that they formed synchronously to
304
the eruption, such as occurred in other volcanic settings (e.g. Campi Flegrei; Vitale and Isaia, 2014;
305
Solfatara volcano; Isaia et al., 2015). Faults as a whole show N-S, E-W, NW-SE and NE-SW main
306
directions, and the T-axis indicate a NNE-SSW extension, well-fitting with kinematic analysis carried
307
out on faults by Bianco et al. (1998).
308
At this point, some considerations have to be done about the relationship between faults, caldera and
309
vent main centers and eruptive fissures. The SV eruptive history was characterized by an alternation
310
of open and closed conduit stages with four caldera collapses (Cioni et al., 1999). However, the
311
caldera and vent main centers were localized in different sectors of the present caldera depicting a
312
triangular area bounded by ca. E-W, N-S and NE-SW sides (Fig. 1). The (i) caldera centers of Mercato
313
(8 ka; Cioni et al. 2008) and Pompeii (79 AD) eruptions; the (ii) vent centers of Mt. Somma volcano
314
and the recent Gran Cono; (iii) the eruptive fissures located in the southern flank of SV and the (iv)
315
dykes within the Gran Cono, are all aligned along a N-S lineament. The N-S and E-W directions are
316
well-marked in the fracture preferred directions located close to the Gran Cono and also by the rose
317
diagram of eruptive fissures (Tadini et al., 2017). All these features suggest that the SV conduit is
318
localized in the area between the intersections of major faults characterized by the well-known
319
Apenninic (NW-SE) and anti-Apenninic (NE-SW) trends and probably by a major N-S fault.
320
Finally, the lineament analysis carried out on a high-resolution DTM, marks a superposition of (i) a
321
clustered pattern, characterized by NNE-SSW and ENE-WSW directions, and (ii) a radial pattern.
322
However, the clear separation between these two configurations, suggest that they were recorded in
15
different stages (e.g. Quintà et al., 2015). As discussed before, the radial pattern is the expression of
324
a closed conduit during a volcano inflation with eventually a subsequent caldera collapse (Fig. 11b,
325
c); on the contrary a clustered pattern could be related to an open conduit (Fig. 11a) characterized by
326
a prevailing unidirectional extension and a very weak radial stress field. In the case of a closed conduit
327
the volcanic stress field (marked by radial pattern) overcome the regional stress field (marked by a
328 clustered pattern). 329 330
6. Conclusions
331This study provides a general framework of deformation at meso-scale of Somma Vesuvio volcano.
332
Different kinds of fractures and faults were observed and measured. Cooling joints occur in lava
333
layers and dykes; metric fractures crosscutting several volcanic deposits generally occur as radial and
334
tangential with respect to the caldera/cone center. Macro-fractures, locally present in the eastern SV
335
flank, are related to rotational landslides. Cooling joints, oriented parallel and orthogonal to the Mt.
336
Somma scarp, were reactivated as normal and oblique faults, respectively. Metric to decametric
high-337
angle normal ring faults bound the caldera rim showing few meters of displacement. Sporadic
338
decametric low-angle normal faults are located in Mt. Somma scarp crosscutting early dykes, in turn
339
deformed by radial and tangential fractures and ring faults. Finally, centimetric, syn-eruptive normal
340
faults occur in pyroclastic deposits located along the SV flanks.
341
Dykes occur as single structures or sets with parallel or en-echelon geometries. Generally, the latter
342
structures are located at the boundaries of structural depression, suggesting a synchronous normal
343
shear component during their development. Orientation analyses highlighted a clustered arrangement
344
for all of the studied structures when analyzed with respect to their spatial location (this latter
345
performed by subdividing the area in cells and in circular areas) or grouped in two temporal classes
346
(older or younger than 79 AD). Results indicate that there are four dominant directions (NW-SE,
16
SW, N-S and E-W). The well-known Apenninic (NW-SE) and anti-Apenninic (NE-SW) trends are
348
prevalent for structures located in older rocks (<79 AD) and in sectors far from the Gran Cono (>2
349
km), while N-S and E-W directions are better recorded in younger rocks mainly localized close to the
350
Gran Cono.
351
The orientation of the dykes and morphological lineaments (the latter ones extracted from DTM)
352
were studied with respect to the old Somma volcano and Gran Cono centers, respectively. These
353
analyses mark a superposition between radial and clustered strain patterns. We envisage that these
354
two dominant patterns are related to different volcanic evolution stages: during a closed conduit, the
355
volcanic inflation forms a uniform strain field, related to the stress localization above the magma
356
chamber center, largely exceeding unidirectional regional strain fields. On the other hand during the
357
open conduit activity, the intensity of radial pattern is very low and the regional strain pattern prevails.
358
In the first case, the volcano inflation produces tangential and radial fractures that, in a possible
359
subsequent caldera collapse stage, can be reactivated as normal and oblique faults, respectively, the
360
latter acting as transfer structures. In the second case, the occurrence of an open conduit allows the
361
development of dykes with horizontal magma flows and eruptive fissures often forming en-echelon
362
sets.
17
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Figure captions:
504 505
Fig. 1. Geological map of the Somma-Vesuvio (ISPRA, 2017). Eruptive fissure data from Santacroce,
506
1987 and Bianco et al. 1998. Vent data from Bianco et al., 1998; Paoletti et al., 2016. Fault (literature)
507
data from Bianco et al. 1998. Caldera and major cone rim data from Cioni et al., 1999. Coordinates
508
are expressed in the UTM WGS84 coordinate system.
509 510
Fig. 2. a) Reactivated cooling joint both radial and tangential; b) plumose structure in lavas; c)
511
orthogonal sub-vertical cooling joint sets in dykes; d) radial fractures meter-length and low angle
512
normal faults hosted along Somma Mt. scarp; e) tangential fractures hosted in Mt. Somma products;
513
f) radial fractures hosted in the Gran Cono crater.
514 515
Fig. 3. a) Satellite image of the macro-fractures in the eastern side of the Somma scarp. b-c) pictures
516
of the macro-fracture. d) Panoramic view of en-echelon dykes; e-f) pictures of the dextral and sinistral
517
en-echelon dyke arrays; g) single dyke with complex geometry; h) two parallel dykes hosted in the
518
1855-1872 AD deposits of Vesuvio.
519 520
Fig. 4. Stereographic projections (lower hemisphere, equal-area net), contour plots and rose-diagrams
521
of fractures and S3 eigenvectors. 522
523
Fig. 5. Maps of a) fracture main directions; b) S3 directions; c) fault main directions and d)
T-524
directions. Rose-diagrams of main directions for fractures included in the e) area 1; f) area 2 and g)
525
area 3. Rose-diagrams of main directions for fractures included in the h) deposits older than 79 AD
526
and i) deposits younger than 79 AD.
527 528
24
Fig. 6. a) Map of analyzed dykes; b) (dyke azimuths) vs diagram (see the text for the explanation);
529
c) histogram of the -frequency; d-e) rose diagram of dyke directions.
530 531
Fig. 7. a) Normal fault plane developing a few centimeter-size damage zone; b-c) tens of meters long
532
high-angle normal faults parallel to the caldera scarp; d) normal fault plane showing slicken-side
533
striations in scoriae; e) syn-sedimentary normal fault with growing strata hosted in pyroclastic
534
deposits.
535 536
Fig. 8. Stereographic projections, contour plots and rose-diagrams of faults and T-axes.
537 538
Fig. 9. a) Criteria for lineaments recognition; b) study area and traced lineaments.
539 540
Fig. 10. (lineament azimuths) vs diagram (see the text for the explanation), histogram of the
-541
frequency and rose diagram of lineament directions calculated for six different circles (centered on
542
the Gran Cono) enclosing areas a) 0-1 km, b) 0.5-1.5 km, c) 1-2 km, d) 1.5-2.5 km, e) 2-3 km and f)
543
3-4 km from the Gran Cono center.
544 545
Fig. 11. Cartoons showing deformation structures formed during: a) opened conduit condition; b)
546
closed conduit condition with volcano inflation; c) closed conduit condition with caldera collapse.
